1. Technical Field
The present invention relates in general to the field of automated solder deposition systems and in particular to an improved method for depositing solder onto solder wettable contact pads on a circuit board. Still more particularly, the present invention relates to a method for accurately maintaining a tool at a desired height above a circuit board or other planar member.
2. Description of the Related Art
Solder distribution onto mounting pads for surface mount boards has generally been accomplished in the semiconductor industry utilizing a screening process. In a screen process artwork and screens must be fabricated having the solder deposition pattern. Then, a precision alignment process is carried out wherein the solder is screened onto the surface mount pads. The solder parts used for this process requires a substantially long cure and bake time. Thus, in addition to the necessary complexity of the alignment process, this prior art technique is relatively time consuming.
The prior art solder distributes technique utilizing screening is further complicated by a requirement that the pattern mix very fine lead pitch and width surface mound pads along with standard surface mount pads. For example, in the case of tape automated bonding, the pitches vary from four to twenty mils, while standard surface mount parts have pitches in the range of twenty to fifty mils. Thus, this process required that different amounts of solder be distributed on various parts of the board. Given the precision required, it is common to utilize separate screening steps, one for very fine lead pitch and width surface mount parts and a second for the standard surface mount parts. There is also a possibility of damaging the solder deposited in a previous step when multiple screening operations are carried out. Additionally, screening fine line solder presents a problem because the solder paste tends to stick in the openings of the screen, as the openings get progressively narrower.
The prior art also discloses other techniques for depositing solder across the surface of a printed circuit. Dip soldering and wave soldering are both techniques which are known in the prior art. Wave soldering involves pumping a molten molder through a nozzle to form a standard wave. In this presence, the entire side of an assembly containing printed conductors with the leads from the circuit components projecting through various points generally travels at a predetermined rate of speed over the standard surface of the wave of molten solder. The lower surface of the assemblies is placed into contact with the upper fluid surface of the wave.
By this technique, the solder wave in the first instance wets the joining surfaces and promotes through hole penetration. This in turn helps to assure the formation of reliable solder joints and fillets. Wave soldering is illustrated in U.S. Pat. Nos. 3,705,457 and 4,360,144. An example of an immersion technique is illustrated in U.S. Pat. No. 4,608,941 wherein panels are immersed in a liquid solder bath and then conveyed to an air knife which levels the molten solder on the panels. The air knife is therefore used to effectively clear the panels of excess solder and only the printed patterns retain the solder.
Another example of a solder leveler is contained in U.S. Pat. No. 4,619,841. The technique disclosed therein is used in conjunction with dip soldering techniques. Other techniques of selective deposition of solder onto printed circuit patterns are described in U.S. Pat. Nos. 4,206,254, 4,389,711 and 4,493,856.
U.S. Pat. No. 3,661,638 is also directed to a system for leveling and controlling the thickness of a conductive material on the walls of through-holes of a printed circuit board. That technique for removing the excess amount of conductive material employs heating to melt a conductive material after it has been deposited. Then, while the conductive material is in a plastic state, gyrating the board to cause the plastic material to move circumferentially about the throughhole and flow axially through the through-hole.
More recently, several techniques have been proposed which utilizes a solder nozzle which deposits solder onto solder wettable contact pads in substantially uniform amounts on each pad. The tool utilized with such a nozzle generally comprises a solder reservoir or plenum, a heating element to melt the solder, and at the bottom of the reservoir, a foot which contains the nozzle and which passes over the contact pads to be wetted with solder. Examples of these types of systems are disclosed in U.S. Pat. No. 4,898,112, filed Apr. 15, 1988, entitled "Solder Deposition System" and U.S. Pat. No. 5,042,708, filed of even date herewith entitled "Solder Placement Nozzle Assembly."
Methods utilized to adjust the height of a solder nozzle, such as those described in the abovereferenced patent applications, are completely manual and typically utilize mechanical shims or feeler gauges of known thicknesses to measure the height between the tip of the solder nozzle and the circuit board or planar member upon which solder is to be deposited. Simple trial and error techniques were then utilized to adjust the nozzle height until it was within a desired range. This technique is quite time consuming and expensive.
A problem which exists with such systems is that the utilization of tools which contact the soldering nozzle and the surface to be soldered generally cause flux to be forced out from between the solder nozzle and the surface to be soldered, resulting in flux covered set up tools and the spreading of flux onto the soldering nozzle and the surface being soldered. Another problem which exits with such systems is that the surface being soldered varies in thicknesses and flatness and therefore tool height needs to be measured at each solder application point.
The environment near a solder nozzle is generally filled with flux, flux vapor, molten solder, solder oxides and solder vapor and as a result attempts at utilizing contact sensors to ensure proper solder nozzle height have been generally unsuccessful.
It should therefore be apparent that a need exists for a method and apparatus which permits the accurate and efficient adjustment of a solder nozzle height in a solder deposition system.